砥川安山岩溶岩中の気泡の形態と分布

抄録

Morphology, abundance and vertical distribution of vesicles were studied in a thick (40-60 m) andesitic lava flow, that lies in the underground of Kumamoto City. The vesicles are frozen bubbles that were fomed in the molten lavas at the time of its eruption. The lava can be divided into three zones: (1) an upper vesicular zone (2) a middle non-vesicular and compact zone and (3) a lower, thinner vesicular zone. The vesicles in the upper zone are elongated vertically, probably due to bouyancy-driven ascent of the bubbles, and those in the lower zone are flattened and elongated horizontally, that may be ascribed to a viscous shear flow at the bottom of the lava flow. Size distribution of the vesicles typically display nearly the log-normal distribution. Abundance, the mean size and the number density of the vesicles are greater in the upper zone than in the lower zone. Such vesicle distribution pattern is consistent to the hypothesis that the lava originally contained abundant bubbles when it was poured on the ground and then the bubbles started to ascent in the lava. Vesicles in the lower zone were the bubbles trapped by the advancing cooling front from the bottom surface of the lava. Bubbles that have escaped from the cold trap below have been accumulated in the upper zone and have been frozen in the lava upon cooling from the top surface. Mass balance calculation, however, indicates that much of the bubbles that were originally present in the lava, have been escaped through the lava surface. A dynamic cooling model was, therefore, proposed, that is to say, in the presence of surface flow in the lave during its cooling, impermeable lava crusts may not be maimtained so that gas bubbles may leak out of the lava into the air.